I looked at the Patty and Bun restaurant chain's website, and a fancy burger is 10.95 pounds, 12.37 USD at the present exchange rate. An equivalent sandwich at the US chain Five Guys in the USA costs $10.81 plus tax. Five Guys doesn't serve booze though. Apparently the average cost of a pint in London is 5.5 pounds right now, so I'm still not seeing anything like a 50 pound bill, unless those chips were flaked with gold or it was a really expensive restaurant.